Solid Inner Core

The meta answer is RESPONSIBLE (Hint: Look for an 11-letter adjective).

Five symmetrically placed Across answers had clues with both an upper-case first letter (which, of course!) and an unusually upper-case last letter as well. (In the original print version of this puzzle, those first and last letters of those clues were subtly bolded).

This might have led you to notice that the first and last upper-case letters of those five clues also appear in their own answers, but with a few extra letters in between them.

Before we get to those in-between letters, let's recap those clues:

17A: "OPEN," "SALE!", and "10 items or lesS"

23A: Under typical conditions, the state water's in when temps are higH

39A: Thing that gets "pulled" in order to deceivE

51A: Elastic properties that allow stretched materials to snap back to normaL

63A: Rich coffee cake with a two-color swirL

From top to bottom, the first upper-case letters spelled OUTER and the last upper-case letters spell SHELL. So we've not got an OUTER SHELL... so what's that to do with an 11-letter adjective?

The puzzle's title, "Solid Inner Core," hints that the answer involves the inside of something. Could there be something within that OUTER SHELL?

Yes, that's where those "in-between" letters come in. Here are the answers, with the "OUTER SHELL" letters in square brackets:

17A: ST[O]RE[S]SIGNS

23A: GASEO[U]SP[H]ASE

39A: FAS[T]ON[E]

51A: R[E]SI[L]IENCIES

63A: MA[R]BLE[L]OAF

Recall the puzzle's title, "Solid Inner Core." From top to bottom, the letters at the core of the grid's "OUTER SHELL" letters spell RESPONSIBLE, which is a synonym for "solid" and an 11-letter adjective that satisfies the puzzle's hint.

I got stuck after OUTER SHELL. Should have seen the next step. I would have If I’d written the right letters next to the themes. Very nice construction!

Took much staring, iterating and collaborating, but aha! found it! Very unique, clever construction

WOW! That could be a Gaffney 5 any day of the week. Very sophisticated (way too sophisticated for me who was looking at ORES and STONE for solidarity. I saw the end letter capped but did not think of taking the first letter as well. Additionally I did not see any except two of the so formatted clues. I'd like to think that if I had I would have seen OUTER SHELL and realized what to do next, but I'd be fooling myself. This was a superbly constructed meta. You have some imagination COry!